FIG. 5(a) shows the configuration of a conventional anesthesia apparatus (refer to Non-Patent Document 1). In FIG. 5(a), the anesthesia apparatus includes: an operation section 51, a control section 52, an anesthetic agent bottle 53, a motor driver 54, a stepping motor 55, a metering pump 56, a vaporizing chamber 59, an anesthetic-agent identifying sensor 61, an anesthetic-agent detecting sensor 62 and a fresh-gas flow-rate sensor 63. An anesthetic agent is stored in the anesthetic agent bottle 53 and is sent through an anesthetic-agent flow path 71 to the suction port of the metering pump 56. A fresh gas is a mixture of oxygen, a nitrous oxide and air and is supplied through a gas pipe line 75 to the vaporizing chamber 59.
Upon setting an anesthetic gas concentration in the operation section 51, the control section 52 calculates a required volume of an anesthetic gas (gaseous matter) on the basis of the set anesthetic-gas concentration value and a fresh-gas flow rate detected by the fresh-gas flow-rate sensor 63. On the basis of Avogadro's law, the control section 52 converts the volume of the anesthetic gas (gaseous matter) into a volume of an anesthetic agent (liquid matter). In order to allow the metering pump 56 to discharge the calculated volume of anesthetic agent, the control section 52 regulates the revolution speed of the motor 55 via the motor driver 54. From the metering pump 56, the anesthetic agent is discharged in the fixed volume corresponding to the set anesthetic-gas concentration value. Then, the anesthetic agent is delivered through a flow path 72 to the vaporizing chamber 59, and in the vaporizing chamber 59, the anesthetic agent is mixed with the fresh gas. This operation is continuously executed, and thereby, even if the set anesthetic-gas concentration value or the fresh-gas flow rate is varied, then the variation is followed by a real-time variation in the volume of liquid delivered from the metering pump 56.
In the conventional anesthesia apparatus, the metering pump 56 needs to deliver a predetermined volume of liquid precisely at an extremely-low flow rate in units of microns. In order to make such a liquid delivery, a metering pump (e.g., see FIGS. 1(a) to 1(d)) is employed which: is driven by the stepping motor 55; includes an eccentric mechanism converting a revolving motion of the stepping motor 55 into a reciprocating motion of a plunger of the metering pump; and makes a constant liquid delivery by sucking and discharging a liquid through variations in the cubic volume of a cylinder of the metering pump caused by the reciprocating motion of the plunger.
When the stepping motor 55 revolves at a constant speed, the reciprocating motion of the plunger in the axial directions becomes a substantially sinusoidal reciprocating motion. Accordingly, as shown in FIG. 5(b), the temporary variation in the suction volume and discharge volume of the metering pump traces a sinusoidal transition. Consequently, during the discharge period, an anesthetic agent is delivered such that the discharge volume varies in a sinusoidal form with time, thereby giving a pulsation to the discharge volume of the metering pump. As shown in FIG. 5(c), the effect of the pulsation remains even though in the vaporizing chamber 59, the anesthetic agent vaporizes gradually to smooth the temporary variation in the anesthetic gas concentration.
FIG. 6(a) shows a result of gas concentration measurements where the anesthetic-gas concentration value is set to 1.0[%] and the fresh-gas flow rate is varied in accordance with the periods: it is 0.5 [L/min] during a period of Y0 to Y1, 1.0 [L/min] during a period of Y1 to Y2 and 6.0 [L/min] during a period of Y2 to Y3. A conspicuous pulsation appears during the period Y0-Y1 when the fresh-gas flow rate is relatively low. Then, the pulsation amplitude lowers during the period Y1-Y2 when the fresh-gas flow rate is doubled, and further, the pulsation becomes almost inconspicuous during the period Y2-Y3 when the fresh-gas flow rate is much higher. In short, the effect of a pulsation appears conspicuously when the fresh-gas flow rate is low and the revolution speed of the metering pump 56 (stepping motor 55) is relatively low.
As described above, a metering pump has the problem of a pulsation, and hence, measures to solve the problem have been taken in various fields including an anesthesia apparatus. For example, Patent Document 1 and Patent Document 2 disclose a technical method in which a plurality of plunger pumps are joined together to adjust the total discharge flow rate to a fixed value.
The document JPH0264273A and US2004/151594A1 disclose plungers having cylindrical pistons which move up and down within a cylinder.